1. Technical Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer and a communication apparatus having the dielectric filter and the dielectric duplexer.
2. Related Art of the Invention
Variable-frequency type dielectric filters such as those using variable-capacitance diodes D11 and D12 shown in FIGS. 11 and 12 have been proposed for designing portable telephone sets smaller in power consumption and in size.
FIG. 11 shows the circuit configuration of a conventional variable-frequency bandpass filter. In the circuit shown in FIG. 11, portion 1 is an input terminal; portion 2 is an output terminal; portion 3 is a voltage control terminal; components 5 and 6 are dielectric resonators; components C21, C22, and C23 are coupling capacitors; components C24 and C25 are capacitors for changing a frequency band; components D11 and D12 are variable-capacitance diodes; and components L11 and L12 are choke coils.
FIG. 12 shows the circuit configuration of a conventional variable-frequency bandstop filter. In the circuit shown in FIG. 1, portion 1 is an input terminal; portion 2 is an output terminal; portion 3 is a voltage control terminal; components 5 and 6 are dielectric resonators; components C26 and C27 are capacitors; component L10 is a coupling coil; components C28 and C29 are coupling capacitors for determining an amount of stop band attenuation; components C24 and C25 are capacitors for changing a frequency band; components D11 and D12 are variable-capacitance diodes; and components L11 and L12 are choke coils.
The dielectric filter thus arranged has a center frequency determined by the resonant frequencies of resonant systems respectively formed of the capacitances of the variable-capacitance diodes D11 and D12, the capacitances of the capacitors C24 and C25, and the dielectric resonators 5 and 6. The capacitances of the variable-capacitance diodes D11 and D12 are changed by changing a voltage applied to the voltage control terminal 3, thus enabling variable setting of the center frequency.
The conventional dielectric filters, however, have a drawback in that, since the variable-capacitance diodes D11 and D12 for variable setting of a center frequency are respectively connected to dielectric resonators 5 and 6 in parallel with the same, a deterioration is caused in Q.sub.o of the resonant systems (Q at the center frequency) by addition of the capacitances of the variable-capacitance diodes D11 and D12 in parallel with the dielectric resonators 5 and 6. If it is necessary to change the frequency of the dielectric filter by a large amount, an increase in the capacitances of the variable-capacitance diodes D11 and D12 is required. In such a case, a deterioration in Q.sub.o of the resonant systems cannot be avoided. In particular, because the insertion loss of the bandpass filter is dependent on Q.sub.o of the resonant system, a deterioration in the electrical characteristics of the bandpass filter is considerable.